Catalytic converters, such as lean NOx traps (LNT) may be used to reduce NOx emissions, such as from engines that may operate lean of stoichiometry. Performance of an LNT to store, release, and reduce NOx may be affected by various operating conditions, including temperature. Specifically, during lower temperature ranges, the LNT may exhibit degraded storage, release, and/or reduction. As such, various approach have been used to locate the LNT at an optimal position along the exhaust path to balance the desire for a fast light-off from cold start operation, as well as reduce over-temperature operation during high loads, for example.
However, in some conditions, either the LNT is placed too far downstream of the engine (resulting in slow light-off) or the LNT is placed too close to the engine (resulting in accelerated degradation). Further, even when a balance may be achieved between these extremes, where the optimal efficiency region of the LNT is utilized as much as possible (e.g., temperature is maintained between approximately 300° C. and 500° C.), there may still be a lower temperature region in which the LNT may be used but with degraded reaction efficiency during release and reduction.
The inventors herein have recognized that it may be possible to retain efficient placement of the LNT, while also addressing low temperature operation. Specifically, the inventors herein have recognized that even when steady state operating temperatures are in a lower temperature region, it is possible to utilize a locally generated exothermic reaction (which may be locally generated, such as on the LNT), during overall rich air-fuel ratio purging operation. The temporary temperature increase during the purge enables more efficient release of stored NOx (thus leaving the LNT more able to store NOx during subsequent lean operation) as well as more efficient reduction of the released NOx. In addition, the temperature of the LNT remains elevated for a portion of the next lean period, which can improve the NOx storage efficiency during that time. In this way, it is possible to perform NOx storage and release/reaction at different local temperatures to achieve improved performance in NOx emission control. In one example of the embodiment, the exothermic reaction may be generated using a split engine cylinder operation, where some cylinders operate rich and other cylinders operate lean, while the overall air-fuel ratio is still rich. In this way, not only is it possible to generate an exothermic reaction during lower temperature conditions, but the engine can operate with increased fuel saving as some cylinders can operate at increased efficiency by operating lean. As another example, the amount of exothermic heat generated during lower temperature purging operation may be adjusted based on catalyst aging to enable improved performance. As still another example, the amount of exothermic heat may be controlled in the split cylinder operation by adjusting the duration and/or extent of the lean operation during the purging. As one non-limiting example, the lower temperature range may be between 200° C. to 300° C. (or 200° C. to 350° C.), where during temperatures above and/or below this range, purging is performed without generating the exothermic reaction with split cylinder operation.